This invention relates to an electrical processing system in a focus detecting device using the principle of a focus adjusting method known as "a double image superimposing system". In this system, two images of an object are shifted in the opposite directions or one of the two images is shifted and the relative displacement of the two images of the object becomes zero when the correct focalization is obtained. Hence, when using this method, an electrical focus detection is achieved by means of a pair of photoelectric conversion element groups. The invention relates to a focus detecting device for a camera, in which the sign of a focus detection output is changed before and after the correct focalization is obtained.
A number of focus detecting devices have been proposed which utilize the variations in spatial frequencies or the variations in contrast of the image of an object. Recently, a variety of focus detecting devices have been provided which utilize the principle of the above-described "double image superimposing system". Some have been put in practical use.
However, most of the conventional focus detecting devices merely utilize the facet of this technique where the focus detection output shows a maximum value, a minimum value or an extreme value when the correct focalization is obtained. Therefore, when the correct focalization is not yet obtained, it cannot be determined whether the direction of focus change is in the proper direction, i.e., whether correct focalization is going to be obtained or has already been obtained and the device is being driven further out of focus. Therefore, in order to obtain the correct focalization, the focus detecting operation must be carried out over the entire focus detection range from infinity to near point for one operation of detecting the position of the correct focalization. Furthermore, since it is difficult to stop the photographing lens immediately when a correct focalization detecting signal is received from the electrical circuit, it is necessary to store the position of the photographing lens at that time instant to return the photographing lens to the position thus stored. Therefore, it is also difficult to instantaneously produce an output indicative of the correct focalization detecting signal by permitting the photographing lens to follow an object moving at high speed.
In addition, although the photographer can be informed whether the correct focalization is obtained or not, it is difficult to inform to the photographer of the state of the photographing lens when the correct focalization is not obtained. That is, the state of the photographing lens when the photographing lens is focussed on a point before the object (hereinafter referred to as "a front focalization" when applicable) and the state of the photographing lens when it is focussed on a point behind the object (hereinafter referred to as "a rear focalization" when applicable) cannot be easily transmitted as an information signal. Thus, only automatic focus detection is performed by a conventional focus detecting device. Furthermore, since the conventional focus detecting device requires a drive circuit of the photographing lens and its electric source, the device is necessarily bulky and intricate.
The reason why the photographer, especially the beginner, has difficulty obtaining proper focus using his eyes resides in not only the fact that the functions of eyes are different among individuals but also in the fact that the display or indication of the correct focalization is effected in an analog mode. If the focalization is indicated digitally, i.e., if the front focalization, the correct focalization and the rear focalization are indicated in a digital mode, also digitally indicated whether or not the correct focalization is obtained, then it is unnecessary to carry out automatic focus detection. Hence, a so-called "focus indicator" is sufficient for the focussing operation.